1. Field of the Invention
This invention relates to a room temperature curable solventless silicone coating composition suitable for use in coating and protecting packaged circuit boards having electric and electronic chips mounted thereon, and a method of protecting the packaged circuit boards from a severe ambient environment.
2. Prior Art
Packaged circuit boards having electric and electronic elements mounted thereon are widely used as electric parts in automobiles, aircraft and the like. It is a common practice to cover such circuit boards or electric and electronic elements with coating compositions based on a resin or high viscosity oil for the purposes of maintaining electrical insulation and protecting the circuit boards from a severe ambient environment where they encounter high temperature, high humidity, water immersion and dust.
The coating compositions as such are difficult to apply because of high viscosity. For the purpose of improving working efficiency and production yield, they are generally diluted with organic solvents such as toluene, xylene and hexane to a sufficient viscosity to enable dipping, flow coating, brush coating or spraying. The diluted compositions are applied to circuit board surfaces and the organic solvents are evaporated off at elevated temperatures or room temperature. The compositions are then cured by heating or irradiating with ultraviolet radiation, thereby forming coatings.
Most of the solvents used are released in the air without recovery, causing environmental pollution and adversely affecting the health of workers. Attempts have been made to recover evaporating solvents without releasing them in the air, but at the sacrifice of economy because an increased capital investment is needed.
As to performance, the conventional coating compositions also have problems. Since cured coatings are very hard, stresses which are induced by thermal expansion and contraction in accordance with ambient temperature changes can be directly transmitted to electronic elements on the circuit board surface, causing a failure of electronic elements. The problem becomes more serious with the recent trend requiring further size reduction of electronic elements.
Addressing the environmental problem, legislation restricting the use of solvents was enacted in many states of the U.S.A. from the late 1980s. Also for protecting the atmosphere of working places from a hygienic aspect, there is a need to have coating compositions of the solventless type.
There are now available several coating compositions which meet these requirements. Developed were coating silicone compositions of the addition reaction type (using platinum catalysts) and UV curing reaction type which need not be diluted with solvents. The silicone compositions of the addition reaction type, however, have the problem that the platinum catalyst can be poisoned by some special materials of which parts mounted on the circuit board surface are made, restraining the compositions from curing. Thus, the compositions are limited in application. The silicone compositions of the UV curing reaction type have the problem that dark zones which cannot receive UV radiation do not cure. Thus, these compositions are inadequate for coating circuit boards having mounted parts of complex profile.
Also room temperature curable silicone coating compositions of the condensation reaction type are now commercially available. They have a viscosity of more than 500 centipoise at 25.degree.. Because of the high viscosity, conventional coating techniques such as dipping, flow coating, brush coating and spraying cannot be employed in coating circuit boards with these coating compositions.